MAE 155B:
Aerospace Engineering Design II
Lecture 5: Propulsion
John T. Hwang
Winter 2019
aircraft and engine manufacturer production lines since 1989.
Historically,
propulsion
contributed
We conclude that fuel
costs alone have nothas
been sufficient
to stimulate increased
aircraft efficiency,
and that improvementshave
in fuel efficiency
due to the introduction
Propulsion
advancements
contributed
Fuel
burn
have
stagnated
since
thethat
90sa CO
of
newhalf
aircraft reductions
have
decreased
over time.
These findings
suggest
about
of
fuel
burn
reductions
built reductions
aircraft from current production lines, not just
aboutstandard
halfthatofapplies
fuelto newly
burn
2
Fuel burn at design range (1960=100) !
to new designs, is most likely to reduce emissions.
100!
1960s!
Annual Improvement !
Period
Seat-km Ton-km!
1960s
2.3%
3.6%!
1970s
0.6%
-0.1%!
1980s
3.5%
2.5%!
1990s
0.7%
0.9%!
post-2000 0.0%
0.3%
1970s!
75!
seat-km!
1980s!
1990s!
ton-km!
post-2000!
50!
25!
1960!
1965!
1970!
1975!
1980!
1985!
1990!
1995!
2000!
2005!
08!
Year!
ABSTRACT 1. AVERAGE FUEL BURN FOR NEW AIRCRAFT, 1960-2008
[Efficiency Trends for New Commercial Jet Aircraft. ICCT, 2009]
(Rutherford
and Zeinali 2009)
2
John T. Hwang (University of California San Diego)
2
Basic thrust analysis
476
Aircraft Design: A Conceptual Approach
v
(Raymer Daniel 1999)
Fig. 13. 1 Simplified thrust analysis model.
While we often write our equations as if the world is a wind tunnel with
the airplane stationary and the air coming at it, the reality is the opposite.
This model represents a generic aircraft propulsion system-a perfect propeller or a very short jet engine-which is actually flying through the air at
velocity Vo and accelerating the air it encounters by a change in velocity
equal to (V- Vo). Subscript zero indicates the freestream condition, and
the mass flow of air passing through the disk is easily found as air density
times velocity times cross-section area S. Newton's equation, redefined
for fluid flows, states that the force produced equals the mass flow rate
times the applied change in velocity, leading to Eq. (13.1).
The rate of useful work done by the propulsion system, called the thrust power
Pt, equals the product of the thrust force and the aircraft velocity [Eq. (13.2)].
The change
kinetic energy (i.e., work) imparted to the fluid by the
John T. Hwang (University of California
San in
Diego)
propulsion system is determined by the difference in fluid velocity.
3
Basic thrust analysis
476
Aircraft Design: A Conceptual Approach
v
(Raymer Daniel 1999)
Fig. 13. 1 Simplified thrust analysis model.
While we often write our equations as if the world is a wind tunnel with
the airplane stationary and the air coming at it, the reality is the opposite.
ConservationThis
ofmodel
momentum—thrust
is equal
to perfect
masspro-flow rate
represents a generic aircraft propulsion
system-a
peller or a very short jet engine-which is actually flying through the air at
times the change
inandspeed
(we’ll
this
Week
8):
velocity Vo
accelerating
the air derive
it encounters
by a in
change
in velocity
equal to (V- Vo). Subscript zero indicates the freestream condition, and
the mass flow of air passing through the disk is easily found as air density
times velocity times cross-section area S. Newton's equation, redefined
for fluid flows, states that the force produced equals the mass flow rate
times the applied change in velocity, leading to Eq. (13.1).
The rate of useful work done by the propulsion system, called the thrust power
Pt, equals the product of the thrust force and the aircraft velocity [Eq. (13.2)].
The change
kinetic energy (i.e., work) imparted to the fluid by the
John T. Hwang (University of California
San in
Diego)
propulsion system is determined by the difference in fluid velocity.
T = ṁ∆V
3
Basic thrust analysis
476
Aircraft Design: A Conceptual Approach
v
(Raymer Daniel 1999)
Fig. 13. 1 Simplified thrust analysis model.
While we often write our equations as if the world is a wind tunnel with
the airplane stationary and the air coming at it, the reality is the opposite.
ConservationThis
ofmodel
momentum—thrust
is equal
to perfect
masspro-flow rate
represents a generic aircraft propulsion
system-a
peller or a very short jet engine-which is actually flying through the air at
times the change
inandspeed
(we’ll
this
Week
8):
velocity Vo
accelerating
the air derive
it encounters
by a in
change
in velocity
equal to (V- Vo). Subscript zero indicates the freestream condition, and
the mass flow of air passing through the disk is easily found as air density
times velocity times cross-section area S. Newton's equation, redefined
for fluid flows, states that the force produced equals the mass flow rate
times the applied change in velocity, leading to Eq. (13.1).
0The rate of useful work done by the propulsion system, called the thrust power
Pt, equals the product of the thrust force and the aircraft velocity [Eq. (13.2)].
The change
kinetic energy (i.e., work) imparted to the fluid by the
John T. Hwang (University of California
San in
Diego)
propulsion system is determined by the difference in fluid velocity.
T = ṁ∆V
T = ṁ(V − V )
where ṁ = ρSV
3
Basic thrust analysis
T = ṁ(V − V0 )
where ṁ = ρSV
John T. Hwang (University of California San Diego)
4
Basic thrust analysis
T = ṁ(V − V0 )
where ṁ = ρSV
Pout = TV0
John T. Hwang (University of California San Diego)
4
Basic thrust analysis
T = ṁ(V − V0 )
where ṁ = ρSV
Pout = TV0
Pout = ṁ(V − V0 )V0
John T. Hwang (University of California San Diego)
4
Basic thrust analysis
T = ṁ(V − V0 )
where ṁ = ρSV
Pout = TV0
Pout = ṁ(V − V0 )V0
Pin =
∂ ∆E
∂t
John T. Hwang (University of California San Diego)
4
Basic thrust analysis
T = ṁ(V − V0 )
where ṁ = ρSV
Pout = TV0
Pout = ṁ(V − V0 )V0
∂ ∆E
∂t
1
1
Pin = ṁV 2 − ṁV02
2
2
Pin =
John T. Hwang (University of California San Diego)
4
Basic thrust analysis
T = ṁ(V − V0 )
where ṁ = ρSV
Pout = TV0
Pout = ṁ(V − V0 )V0
∂ ∆E
∂t
1
1
Pin = ṁV 2 − ṁV02
2
2
1
Pin = ṁ(V + V0 )(V − V0 )
2
Pin =
John T. Hwang (University of California San Diego)
4
Basic thrust analysis
T = ṁ(V − V0 )
where ṁ = ρSV
Pout = TV0
Pout = ṁ(V − V0 )V0
∂ ∆E
∂t
1
1
Pin = ṁV 2 − ṁV02
2
2
1
Pin = ṁ(V + V0 )(V − V0 )
2
Pin =
η=
Pout
2
=
Pin
V /V0 + 1
John T. Hwang (University of California San Diego)
4
Basic thrust analysis
η=
2
Pout
=
Pin
V /V0 + 1
T = ṁ(V − V0 )
where ṁ = ρSV
John T. Hwang (University of California San Diego)
5
Basic thrust analysis
η=
2
Pout
=
Pin
V /V0 + 1
T = ṁ(V − V0 )
I
where ṁ = ρSV
We see that V > V0 in order to generate thrust.
John T. Hwang (University of California San Diego)
5
Basic thrust analysis
η=
2
Pout
=
Pin
V /V0 + 1
T = ṁ(V − V0 )
where ṁ = ρSV
I
We see that V > V0 in order to generate thrust.
I
As V → ∞, η → 0.
John T. Hwang (University of California San Diego)
5
Basic thrust analysis
η=
2
Pout
=
Pin
V /V0 + 1
T = ṁ(V − V0 )
where ṁ = ρSV
I
We see that V > V0 in order to generate thrust.
I
As V → ∞, η → 0.
I
As V → V0 , η → 1.
John T. Hwang (University of California San Diego)
5
Basic thrust analysis
η=
2
Pout
=
Pin
V /V0 + 1
T = ṁ(V − V0 )
where ṁ = ρSV
I
We see that V > V0 in order to generate thrust.
I
As V → ∞, η → 0.
I
As V → V0 , η → 1.
I
Therefore, we want V to be as small (close to V0 ) as possible
for maximum propulsive efficiency.
John T. Hwang (University of California San Diego)
5
Basic thrust analysis
η=
2
Pout
=
Pin
V /V0 + 1
T = ṁ(V − V0 )
where ṁ = ρSV
I
We see that V > V0 in order to generate thrust.
I
As V → ∞, η → 0.
I
As V → V0 , η → 1.
I
Therefore, we want V to be as small (close to V0 ) as possible
for maximum propulsive efficiency.
I
This means that, for the same amount of thrust T , S must be
as large as possible.
John T. Hwang (University of California San Diego)
5
AE481 — Martins
Sunday 13th November, 2011 at 23:22
71
196
5 Aircraft Engines and Propulsion
Fuel-based propulsion—4 types for subsonic
map
278
Aircraft Design: A Conceptual Approach
Figure 5.8 Figure
Classification
of engine concepts
mostly
used in aviation.
11.1: Classification
of propulsion
systems
Burner
Compressor
I
Turbine
Burner
Compressor
its lower density when compared to kerosene requires
that isI
\ a tank capacity
\
four times larger for flying the same distance. Furthermore, the production
Figure
5.8 Figure
Classification
of
engine
concepts
mostly
used
in
aviation.
and distribution
will
require
the
development
of
new
infrastructure.
11.1: Classification of propulsion systems
1. Piston
Piston-prop
Centrifugal turbojet
Turbine
I
Axial-flow turbojet
its lower 5.2
density
when compared
to kerosene
requires a tank capacity that is
Fundamentals
of reaction
propulsion
four times larger for flying the same distance. Furthermore,
the production
Bypass air
Flameholders
Engine will
concepts
and distribution
require the development of new infrastructure. Fuel spray
barsl
Figure 5.8 Figure
Classification
of
engine
concepts
mostly
used
11.1: Classification of propulsion systems in aviation.
Turbojet
<
Figure 5.8 shows a schematic overview of the most important engine orcon- <0
turbofan
<
cepts used in aviation. Except the rocket engine, all of them are air breathing
lower
density
compared
to kerosene
requires
a tankcategories
capacity
that is
5.2 its
Fundamentals
of
propulsion
engines
that canwhen
be reaction
used
for Turboprop
atmospheric
flight only.
The following
Turbofan
Afterburner
four are
times
larger for flying the same distance. Furthermore, the production
distinguished:
and distribution
will require the development
new infrastructure.
Fig. 10.1 of
Propulsion
system options.
• Piston engines.
Engine
concepts
2. Turbojet
3. Turbofan
• Turboprop and turboshaft engines.
•
Turbojet engines, also called straight or simple jet engines.
of the
engine andof
its the
inlet most
duct orimportant
propeller. Also,
the fuelconsystem must be
Figure 5.8
shows a schematic
overview
engine
• Turbofan engines. defined, especially the fuel tanks that carry a large fraction of the total
Fundamentals
of reaction
propulsion
cepts5.2
used
aviation.
Except
the rocket
• inRamjet
engines.
aircraft
weight. engine, all of them are air breathing
This chapterflight
treats the
integration
and layout of
the propulsion system
engines that can be used for atmospheric
only.
The following
categories
This scheme is not only
based
on mechanical
differences
betweenofpropeller
the overall
vehicle design.
The calculation
installed propulsion perinto
Engine
concepts
are distinguished:
and jet
propulsion, but also the energy conversion process has been taken
formance is covered in Chapter 13 .
4. Turboprop
into account. The different types will be discussed in the following sections,
• Piston
engines.
Figure 11.2: Diagrams showing the components for the turbojet, turboprop and turbofan; note
that all three have a gas generator in common.
which
also adiscuss
the technological
implementation
of the firstengine
four conFigure
5.8 will
shows
schematic
overview of
the most important
• Turboprop
and
turboshaft
engines.
categories
mentioned
above.
Gas turbine engines
with reheat
and ramjet enPropulsion
Overview
and
Selection
cepts
used
in
aviation.
Except
the
rocket
engine,
all
of
them
are
air
breathing
• Turbojet
also called
straight
or simple
jet engines.
ginesengines,
will be discussed
in Chapter
9. Besides
the engine
types mentioned in
10.1 illustrates
the only.
major The
options
for fuel-based
aircraft propulJohn T. Hwang (University
of California
Diego)
engines
that
can San
be used
forFigure
atmospheric
flight
following
categories
• Turbofan
engines.
sion. These all operate by compressing outside air, mixing it with fuel, burnare distinguished:
.m
6
because they provide more thrust per unit power, but electric fans are also
in use. Electric propulsion is discussed at the end of this chapter.
Fuel-based propulsion
(Typical applications)
/
u
u..
..---
Vl
0\
c
·v;
/
"'u
/
c
--- ---
Rocket
? Sc ramjet
Ramjet
Afterburning turbojet
Afterburning low-bypass-ratio turbofan
Low-bypass-ratio turbofan
1 High-bypass-ratio turbofan
1=7" Turboprop
l=7 Piston-prop
0
2
3
4
5
6
Design Mach number
John T. Hwang (University of California San Diego)
(Raymer
Daniel 1999)
7
Piston engine
First type of aircraft propulsion used:
I
Similar to automobile engine
I
But air-cooled, lighter, more reliable
I
Efficient, cheap; but heavy, noisy
I
Larger props more efficient, but heavier and require more
clearance
John T. Hwang (University of California San Diego)
8
Piston engine
The piston engine is limited to subsonic speeds:
I
Limited by helical tip speed due to the possibility of shocks
I
Blade angle of attack decreases as forward speed increases
I
High pitch causes stall at low speeds
I
This motivates variable-pitch props
John T. Hwang (University of California San Diego)
9
Piston engine
The piston engine is constant-power:
Power:
I Proportional to RPM
I
Constant in velocity
Linearly increases with density
Brake-specific fuel consumption (BSFC)
I Fuel burn rate per unit power
I
I
Constant in velocity and density
John T. Hwang (University of California San Diego)
10
Turbine engines
Turbine engines are constant-thrust:
I
Four steps: compress air; mix it with fuel; burn the mixture;
extract energy from the high-pressure hot gas
I
Pros: can operate at higher speeds
I
Cons: not as efficient, especially at low speeds (small, fast
exhaust)
John T. Hwang (University of California San Diego)
11
Fuel-based propulsion
Turbojet engines:
I Compressor: compress air to many times atmos. pressure.
I Burner: inject fuel, mix it with air, and ignite the mixture.
I Turbine: before expelling the hot gas, pass it through a turbine to drive
the compressor.
Turboprop engines:
I Idea: add conventional propeller to turbojet.
I Prop-fan or unducted fan: advanced propellers—loud.
I Open rotor: turbofan-like props—efficient but loud.
Turbofan engines:
I Idea: turbine drives a ducted fan. Some of the air bypasses the core, and
the rest is ducted into the turbojet core.
I Quieter: the duct suppresses noise, and the thrust from the bypassed air
enables a reduced exit velocity because of reduced strength of
noise-generating shear layers.
John T. Hwang (University of California San Diego)
12
(TSFC) — is the fuel consumed by unit time for each unit of thrust. In English units, SFC is in
pounds of fuel per hour, per pound of thrust. SFC varies with Mach number, throttle setting and
altitude. Fig. 2.2 show the typical variation of SFC with Mach number for various types of engines.
Fuel-based propulsion
TSFC vs. Mach Number
John T. Hwang (University of California San Diego)
Figure 2.2: SFC vs. Mach number for various engines Mattingly [2, Fig. 1.17b]
13
Electric propulsion
I
No emissions, and more than 90% energy efficiency possible
versus only 20% for gas engines
I
Fewer moving parts than reciprocating or turbine engines, so
greater reliability and simplicity
I
Smooth and quiet
I
Power does not decrease with altitude
I
Can be overpowered for a short time for takeoff or emergency
conditions
I
Gearbox is not needed
John T. Hwang (University of California San Diego)
14
The blade angle of attack is a
key
aspect of the
propellor design
Propeller-driven
aircraft
9.2
Propeller
Tip
Voo
I
Section A-A
Views
looking
down
from the
top
r
L_
Root
Voo
Side view
Section B-B
Figure 9.3 Illustration of propeller, showing variation of pitch along the blade.
John T. Hwang (University of California San Diego)
15
p
Propellor-driven
aircraft
are
shaft of the engine)
and PAis the
power available from the propeller. As giv
Propeller-driven
aircraft
Eq. (6.31), PA = TA Voo. Hence Eq. (9.2) becomes
fundamentally limited in speed
TAVoo
9.2 Propeller
17=----p
where P is the shaft brake power (the power delivered to the propeller b
C H A PT E R 3
•
Some Propulsion Characteristics
157
7)
rw
As previously explained, TA in Eq. (9.3) is basically an aerodynamic
1.0 - - - - - - - - - - - - - - - - - - - - - - - - nomenon that is dependent on the angle of attack a in Fig. 9.5. In turn
dictated by the pitch angle fJ and ¢, where ¢itself depends
> >on the magnitud
V and rwrOJ. The angular velocity OJ= 2;r n, where n is the number of pro
revolutions per second. Consequently, TA must be a function of at least f
and n. Finally, the thrust must also depend on the size of the propeller, char
ized by the propeller diameter D. In turn, the propeller efficiency, from Eq.
must depend on fJ, Voo, 17, and D. Indeed, theory and experiment
Voo both show
J=(b)
nD
for a fixed pitch angle fJ, 17 is a function of the dimensionless quantity
00
Velocity and relative wind diagrams For a section of
a revolving propeller: (a) Case For low V00 and (b)
case for high V00 •
Advance ratio
Figure 9.6 Propeller efficiency versus advance ratio. Note that D denotes
J= Voo
propeller diameter.
advance ratio
nD
Propeller
These losses occur because of several different
effects. First imagine that you
RPM
typical
variation
17 with
J for
is no
sketched
Fig.
9 .6; three c
are
standing
in anof
open
field. The
airaisfixed
still;diameter
it has
velocity. in
Then
a propeller{3. The angle of attack a is theAangle
between
the chord
wind. The angle of attack clearly
on the
relativegoes zooming
driven
vehicle
by you.different
After the propeller
haspitch.
passed,Figure
you will 9.6
feel is im
aredepends
shown
corresponding
to three
values of
ig. 3.6a, V00 is small, and a is a fairly large
positive
value,
afrom
stiff
breeze
moving in17 the
of the vehicle.
This breeze
tant;
curves
is direction
obtainedopposite
for anthat
airplane
performance
analys
c "lift" L acting in the general thrust
direction.
In such
Fig. 3.6b,
is part of the slipstream from the propeller; that is, the air is set into both transly increased; all other parameters
remain the same.
Here,6.
described
inCh.
lational
motion by the passage of the propeller. Consequently,16
Hwang
of California
Sanand
Diego)
ved John
to theT.other
side(University
of the airfoil
section,
giving
riserotational
to
Examine
Fig.
more closely.
Note that
17 < energy
1; this of
is the
because
some
you observe
some
and rotational
kinetic
air where
dynamic lift force L pointing in the opposite
direction
of 9.6translational
fJ
Computing thrust and torque
Fig.22 Propeller
Propeller Blade
Geometry
Fig.
Blade
Geometry
Fig. 3
and
Scatterplot of the surrogate model inputs
Lee, and
Fig.(Ha,
3 Scatterplot
of theHwang
surrogate2019)
model inputs
CQ =
John T.
Q
1
2
2 ⇢V Sr
,
(5)
where ⇢ is equal to air density, V is the airspeed, S is the propeller blade area, and r is the propeller blade radius.
and
The SMT is trained with these dependencies and outputs and is visualized in Fig. 4, accurately portraying how as
Q must decrease accordingly by increasing RPM, thus
blade angle
of attack increases
a fixed pitch angle,Cinflow
velocity
Hwang (University
of California
San atDiego)
,
Q =
2 Sr
increasing both torque and thrust. Separate design variables 1are
for each propeller while mirrored pairs of propellers
⇢Vset
2
(5)
17
1.0
1.0
0.8
0.8
Thrust coefficient
Torque coefficient
Computing thrust and torque
0.6
0.4
0.2
Blade angle
of attack ( )
=3
=2
=1
=0
= -1
= -2
= -3
0.6
0.4
0.2
0.0
0.0
5
Fig. 4
10
15
20
Blade pitch angle ( )
25
5
10
15
20
Blade pitch angle ( )
25
Lee,
andblade
Hwang
2019)
Coefficient of torque (Ha,
and thrust
versus
pitch angle
at several blade angles of attack.
since dCMi / d↵ = 0, and thus neutral point can be calculated by
1
T = CT ρV 2 S
2
x
np
1
and Q = CQ ρV 2 Sr
2
Õ dCLi
Si xaci
i
d↵
=
.
Õ dCLi
Si
i
d↵
(8)
Using the neutral point and the center of gravity we can calculate the static margin, which we add as a constraint to
ensure longitudinal stability.
John T. Hwang (University of California San Diego)
18
References i
Ha, Tae Hyun, Keunseok Lee, and John T Hwang (2019). “Large-scale design and
economics optimization of eVTOL concepts for urban air mobility”. In: 2019
AIAA/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials
Conference. doi: 10.2514/6.2019-1218.
Raymer Daniel, P (1999). “Aircraft Design: A conceptual approach”. In: AIAA
Education series.
Rutherford, Daniel, Mazyar Zeinali, et al. (2009). “Efficiency trends for new
commercial jet aircraft 1960-2008”. In:
John T. Hwang (University of California San Diego)
19